1. Field of the Invention
This invention pertains to devices for the frequency conversion of the energy in the coherent radiation emitted from lasers. The devices utilize electronic, vibrational or rotational Raman scattering to obtain efficient frequency conversion of the energy. More particularly, this invention pertains to the optical configuration of such devices.
2. Description of the Prior Art
U.S. Pat. No. 4,280,109 ("Stappaerts") described a device for translating the frequency of the energy contained in a laser beam by utilizing higher order Strokes/Raman scattering. In Stappaerts, a first Raman cell, referred to therein as the Raman oscillator cell, is used to generate Stokes radiation, i.e., a Stokes beam, when "pumped" by a portion of the laser beam. The Stokes beam is then spatially filtered anad passed through a second Raman cell referred to therein as the Raman amplifier cell. The second cell amplifies the Stokes beam and, by the careful selection of the dimensions and operating parameters, concentrates most of the translated energy into a single order of Stokes radiation. The second Raman amplifier cell is "pumped" by a portion of the same laser beam but at a lower pumping energy density.
The disclosure of the Stappaerts patent (see colum 3, lines 58-63) also suggests that the Raman amplifier cell and the Raman oscillator cell could be combined into a single Raman cell having four ports, the path within the cell between the first and second ports functioning as the Raman oscillator cell and the path between the third and fourth ports functioning as the Raman amplifier cell. The use of a single cell, however, is not further described or depicted in the specification.
Separate oscillator and amplifier Raman cells, or a single Raman cell having four ports, may be suitable for use when a gas is used as the translation medium within the cell. However, when metal vapors are used as the translation medium, very high temperature ovens must be used to obtain the metal vapor. As a consequence, it becomes important to have a compact configuration in order to minimize the heating requirements and heat loss, which obviates the feasibility of either employing separate Raman cells or a single cell having four ports.